Process of solidifying and remelting zinc



United. St tes P t 2,805,934 7 PROCESS onsormng lfio AND REMELTING No Drawing. v Application June 22, 1953,

i Serial No; 363,369

3 Claims. (ems-s This invention relates to an improved method of handling a metal or alloy in an uncontaminated state during melting, freezing, and vaporization operations. In a specific instance, this invention relates to an improved method of handling high purity zinc during phase transition.

The prevention of contamination of metals at elevated temperatures has created problems which are not easily solved except by the use of unusual materials of construction. Zinc in the molten state is a corrosive solvent for many materials of construction which might otherwise be used in the making of containing vessels. Accordingly, processes which make use of such reagents are accompanied by the indicated problems and out-ofthe-ordinary methods of carrying out the operations have been devised.

The method of this invention may be illustrated by discussing one application wherein its usefulness has been demonstrated. Hyperpure silicon, essentially spectrographically free of metallic impurities and with the low electrical conductivity of interest for semi-conductor electronic devices, has been prepared commercially by the reduction of silicon tetrachloride with zinc vapor. This reduction reaction takes place at 950 C., and is usually carried out in apparatus constructed of fused silica. A suitable apparatus consists of the following inter-connected units: (1) silicon tetrachloride vaporizer and heater, (2) zinc boilers and heaters, (3) a cylindrical tube reactor, and (4) a by-product collection system. A drawing of such an apparatus and the dimensions are included in the publication entitled, Preparation of Hyperpure Silicon by D. W. Lyon, C. M. Olson, and E. D. Lewis, Journal of the Electrochemical Society, volume 96, #6, pages 359-363, December 1949.

The zinc used by the authors of this paper was of 99.99% purity, and was purchased from a commercial producer in 45 lb. slabs. These were cut into small pieces approximately 3 x 1.5 X 1 inches in size. Cleaning was accomplished by allowing the zinc to stand for 20-30 minutes in 20% hydrochloric acid, removing any loose dirt, rinsing with water, and drying. The silicon tetrachloride of C. P. grade was purchased and redistilled to improve its quality.

In order to insure a more closely regulated zinc vaporization rate in this process, two zinc boilers in parallel may be utilized. Zinc boilers of one liter capacity and constructed of transparent silica in a form similar to distillation flasks are suitable for small scale production and may be used as described in the above mentioned publication. During operation, whenever the level in either zinc boiler drops to about A full, more zinc is added through a U tube inlet to the boiler, and at the same time more heat is applied to the other boiler in order to maintain a constant vaporization rate. When the reaction was to be terminated by the reporting investigators all of the zinc in the boilers was vaporized, zinc in the U traps 2 was blown into the boilers by a stream of nitrogen gas, and then this zinc was boiled out.

If for any reason, it became necessary to stop the operation while there was still zinc remaining in the flasks, the flasks invariably broke upon solidification and remelting of zinc. This, of course, prevented further operation until new flasks were installed.

This invention has as one object an improved method of handling a pure metal during its transition from one physical state to another. A further object is to increase the life of boilers used in the conversion of zinc to its vaporous state. A specific object is to provide a method wherein it is possible to allow metallic zinc to freezewithin a silica boiler and to be remelted without destruction of the equipment.

The above and other objects are realized by the following invention which broadly comprises an improved method of handling zinc during phase transition, i. e melting, freezing, and vaporization operations, in vessels con: structed of a material corrosion resistant to molten and vaporo us zinc which comprises carrying out these operations in the presence of elementary silicon. In a more specific and preferred embodiment, this invention comprises adding elemental silicon, in an amount from .l% to about 10% by weight of the zinc charged, to a silica container prior to transition of the zinc from the liquid to the solid state.

To illustrate the workings of this invention the following discussion is given and is meant in no way in limitation thereof.

Example The process of my invention is exemplified by reduction methods such as that mentioned above wherein zinc vapors are generated in a silica apparatus and combined with silicon tetrachloride vapors for the purpose of producing elementary silicon of high purity. In a laboratory scale of operation, zinc boilers of one liter size were loaded with about 3,500 grams each of molten zinc. Ten grams of elementary silicon was added to each flask before addition of the Zinc. This permitted melting, solidification and remelting of the zinc in the flasks as desired without flask breakage for long periods of time, after which the flasks normally failed by devitrification of the silica. Experiments done in the same manner without the silicon in the flasks resulted in destruction of the flasks on freezing of the zinc and remelting.

In the operation of the silicon production unit is was found that immediately upon utilizing the method of this invention the failure of zinc boilers, caused by freezing of the zinc metal, ceased. Also the use of this invention allowed the operator to stop the process at any point desired and the zinc remaining in the boilers did not have to be removed thereby resulting in a saving of time and money.

The amount of elementary silicon utilized does not seem to be an important variable and a range of useful amounts tried is from 0.1% to about 10% of the weight of the zinc charge. An amount within the range of 1% to 5% is considered preferable. Larger amounts of silicon may be used but seem to add no economic advantages. Silicon of high purity is preferred because it is more readily wet by the zinc and more easily dissolved.

In addition to the above use in the preparation of silicon metal, the method might be utilized to produce molten zinc for any purpose as well as zinc vapor for reaction with oxygen or sulphur to prepare pure oxides, sulfides, or other zinc compounds, for phosphors or semiconductors. One might visualize the use of my process to allow zinc to be used as a heat transfer fluid in silica equipment or permit storage of the zinc in a silica vessel Patented Sept. 10, 1957.

. 3 from which it may be remelted or vaporized without breakage of the containing vessel.

The exact nature of the action of the silicon in the molten zinc-during the melting and freezing operation is not known, but it might be surmised that the presence of the silicon prevents the freezing of .the zinc to the walls of the silica fiasksso that all tensions between zinc and silica are removed during the freezing and remelting of the zinc. The literature states that the solubility of silicon in molten zinc varies greatly with the temperature, it being much more soluble at the boiling point of the zinc (907 C.) than at its melting point (420 C.)'. It follows that when molten zinc is heated in the presence of solid elemental silicon there will be solution of silicon and a separation of silicon will occur on cooling if the solution were saturated at the higher temperature. It is understood that solid silicon may be present when phase transition of zinc between the liquid and solid states occurs during the practice of my invention. Silicon in large amounts, i. e., greatly in excess of its solubility, is unnecessary and the benefits of my invention are obtained when the zinc in the molten state and about to be frozen contains dissolved silicon and the benefits of the inven tion are also realized when silicon in excess of its solubility is present in the solid state.

I claim as my invention:

1. In a process for shutting down and subsequently starting up a vapor phase reduction of vaporous silicon tetrachloride with zinc vapor .to produce silicon, said vapor phase reduction being carried out in a silica apparatus comprising the following interconnected units: (1) a silicon tetrachloride vaporizer and preheater (2) zinc boiler and preheater (3) a reduction reactor and (4) a byproduct collection system, an improvement which prevents the destruction of the zinc boiler due to solidification and remelting of the zinc upon shutting down and subsequently starting up the reduction process-comprising maintaining in the boiler during solidification and remelting 0.1% to 10% of silicon based on the weight of the zinc.

2. A process for solidifying and remeltingzinc in a silica container without destruction of said container com prising maintaining in the zinc during solidification 0.1% to 10% of silicon, based on the Weight of zinc, and subsequently heating the zinc and silicon in said silica container at least to the melting point of the metal.

3. A process for solidifying and remelting zinc in a silica container without destruction ,of said container comprising maintaining in the zinc during solidification an amount of silicon sufficient to prevent destruction of said silica container on subsequent heating, and then heating the zinc and silicon in said silica container at least to the melting point of the metal.

References Cited in the file of this patent A Comprehensive Treatise on Inorganic and Theoretical Chemistry by Mellor, vol. VI, page 182, 1925. 

3. A PROCESS FOR SOLIDIFYING AND REMELTING ZINC IN A SILICA CONTAINER WITHOUT DESTRUCTION OF SAID CONTAINER COMPRISING MAINTAINING IN THE ZINC DURING SOLIDIFICATION AN AMOUNT OF SILICON SUFFICIENT TO PREVENT DESTRUCTION OF SAID SILICA CONTAINER ON SUBSEQUENT HEATING, AND THEN HEATING THE ZINC AND SILICON IN SAID SILICA CONTAINER AT LEAST TO THE MELTING POINT OF THE METAL. 